Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g., parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE (Release 8)
The downlink component carrier of a 3GPP LTE (Release 8) is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE (Release 8) each subframe is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g., employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g., to fulfill the requirements on IMT-Advanced. Two major technology components are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers (component carriers) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE are in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanism (e.g., barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between center frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively. The transport channels are described between MAC and Layer 1, the logical channels are described between MAC and RLC.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g., TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment; the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC), while in the uplink it is the Uplink Primary Component Carrier (UL PCC).
Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to an SCell is a Downlink Secondary Component Carrier (DL SCC), while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).
The characteristics of the downlink and uplink PCell are:
                For each SCell the usage of uplink resources by the UE in addition to the downlink ones is configurable (the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs, and no SCell can be configured for usage of uplink resources only)        The downlink PCell cannot be de-activated, unlike SCells        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        Non-access stratum information is taken from the downlink PCell        PCell can only be changed with handover procedure (i.e., with security key change and RACH procedure)        PCell is used for transmission of PUCCH        The uplink PCell is used for transmission of Layer 1 uplink control information        From a UE viewpoint, each uplink resource only belongs to one serving cell        
The configuration and reconfiguration of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover). In other words, while in connected mode, UEs need not acquire broadcast system information directly from the SCells.
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
LTE RRC States
LTE is based on only two main states: “RRC_IDLE” and “RRC_CONNECTED”.
In RRC_IDLE the radio is not active, but an ID is assigned and tracked by the network. More specifically, a mobile terminal in RRC_IDLE performs cell selection and reselection—in other words, it decides on which cell to camp. The cell (re)selection process takes into account the priority of each applicable frequency of each applicable Radio Access Technology (RAT), the radio link quality and the cell status (i.e., whether a cell is barred or reserved). An RRC_IDLE mobile terminal monitors a paging channel to detect incoming calls, and also acquires system information. The system information mainly consists of parameters by which the network (E-UTRAN) can control the cell (re)selection process. RRC specifies the control signaling applicable for a mobile terminal in RRC_IDLE, namely paging and system information. The mobile terminal behavior in RRC_IDLE is specified in TR 25.912, e.g., Chapter 8.4.2 incorporate herein by reference.
In RRC_CONNECTED the mobile terminal has an active radio operation with contexts in the eNodeB. The E-UTRAN allocates radio resources to the mobile terminal to facilitate the transfer of (unicast) data via shared data channels. To support this operation, the mobile terminal monitors an associated control channel which is used to indicate the dynamic allocation of the shared transmission resources in time and frequency. The mobile terminal provides the network with reports of its buffer status and of the downlink channel quality, as well as neighboring cell measurement information to enable E-UTRAN to select the most appropriate cell for the mobile terminal. These measurement reports include cells using other frequencies or RATs. The UE also receives system information, consisting mainly of information required to use the transmission channels. To extend its battery lifetime, a UE in RRC_CONNECTED may be configured with a Discontinuous Reception (DRX) cycle. RRC is the protocol by which the E-UTRAN controls the UE behavior in RRC_CONNECTED.
FIG. 7 shows a state diagram with an overview of the relevant functions performed by the mobile terminal in IDLE and CONNECTED state.
Logical and Transport Channels
The MAC layer provides a data transfer service for the RLC layer through logical channels. Logical channels are either Control Logical Channels which carry control data such as RRC signaling, or Traffic Logical Channels which carry user plane data. Broadcast Control Channel (BCCH), Paging Control channel (PCCH), Common Control Channel (CCCH), Multicast Control Channel (MCCH) and Dedicated Control Channel (DCCH) are Control Logical Channels. Dedicated Traffic channel (DTCH) and Multicast Traffic Channel (MTCH) are Traffic Logical Channels.
Data from the MAC layer is exchanged with the physical layer through Transport Channels. Data is multiplexed into transport channels depending on how it is transmitted over the air. Transport channels are classified as downlink or uplink as follows. Broadcast Channel (BCH), Downlink Shared Channel (DL-SCH), Paging Channel (PCH) and Multicast Channel (MCH) are downlink transport channels, whereas the Uplink Shared Channel (UL-SCH) and the Random Access Channel (RACH) are uplink transport channels.
A multiplexing is then performed between logical channels and transport channels in the downlink and uplink respectively.
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data-related information (e.g., HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling needs only be transmitted once per TTI.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). A PDCCH carries a message as a Downlink Control Information (DCI), which includes resource assignments and other control information for a mobile terminal or groups of UEs. In general, several PDCCHs can be transmitted in one subframe.
It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
With respect to scheduling grants, the information sent on the L1/L2 control signaling may be separated into the following two categories, Shared Control Information (SCI) carrying Cat 1 information and Downlink Control Information (DCI) carrying Cat 2/3 information.
Shared Control Information (SCI) Carrying Cat 1 Information
The shared control information part of the L1/L2 control signaling contains information related to the resource allocation (indication). The shared control information typically contains the following information:                A user identity indicating the user(s) that is/are allocated the resources.        RB allocation information for indicating the resources (Resource Blocks (RBs)) on which a user(s) is/are allocated. The number of allocated resource blocks can be dynamic.        The duration of assignment (optional), if an assignment over multiple sub-frames (or TTIs) is possible.        
Depending on the setup of other channels and the setup of the Downlink Control Information (DCI)—see below—the shared control information may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Downlink Control Information (DCI) Carrying Cat 2/3 Information
The downlink control information part of the L1/L2 control signaling contains information related to the transmission format (Cat 2 information) of the data transmitted to a scheduled user indicated by the Cat 1 information. Moreover, in case of using (Hybrid) ARQ as a retransmission protocol, the Cat 2 information carries HARQ (Cat 3) information. The downlink control information needs only to be decoded by the user scheduled according to Cat 1. The downlink control information typically contains information on:                Cat 2 information: Modulation scheme, transport-block (payload) size or coding rate, MIMO (Multiple Input Multiple Output)-related information, etc. Either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated resource blocks)        Cat 3 information: HARQ related information, e.g., hybrid ARQ process number, redundancy version, retransmission sequence number        
Downlink control information occurs in several formats that differ in overall size and also in the information contained in its fields. The different DCI formats that are currently defined for LTE are as follows and described in detail in 3GPP TS 36.212, “Multiplexing and channel coding”, section 5.3.3.1 (available at http://www.3gpp.org and incorporated herein by reference).
Format 0:
DCI Format 0 is used for the transmission of resource grants for the PUSCH.
For further information regarding the DCI formats and the particular information that is transmitted in the DCI, please refer to the technical standard or to LTE—The UMTS Long Term Evolution—From Theory to Practice, Edited by Stefanie Sesia, Issam Toufik, Matthew Baker, Chapter 9.3, incorporated herein by reference.
Downlink & Uplink Data Transmission
Regarding downlink data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH), along with the downlink packet data transmission. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the data is transmitted (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the mobile terminal (receiver) to identify the resources on which the data is transmitted.        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier (“cross-carrier scheduling”). This other, cross-scheduled component carrier could be for example a PDCCH-less component carrier, i.e., the cross-scheduled component carrier does not carry any L1/L2 control signaling.        
The Transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g., the number of resource blocks assigned to the user equipment)) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. The modulation scheme may be signaled explicitly.                Hybrid ARQ (HARQ) information:                    HARQ process number: Allows the user equipment to identify the hybrid ARQ process on which the data is mapped.            Sequence number or new data indicator (NDI): Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a PDU prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).                        UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the user equipment should transmit the data (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier. This other, cross-scheduled component carrier may be for example a PDCCH-less component carrier, i.e., the cross-scheduled component carrier does not carry any L1/L2 control signaling.        L1/L2 control signaling for uplink grants is sent on the DL component carrier that is linked with the uplink component carrier or on one of the several DL component carriers, if several DL component carriers link to the same UL component carrier.        The Transport Format, the user equipment should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g., the number of resource blocks assigned to the user equipment)) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    HARQ Process number: Tells the user equipment from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a protocol data unit (PDU) prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
There are several different possibilities how to exactly transmit the information pieces mentioned above in uplink and downlink data transmission. Moreover, in uplink and downlink, the L1/L2 control information may also contain additional information or may omit some of the information. For example:                HARQ process number may not be needed, i.e., is not signaled, in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed, and thus not signaled, if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre-defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as, e.g., pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (on the Physical Uplink Shared Channel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore, it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e., the RV info is embedded in the transport format (TF) field. The Transport Format (TF) respectively modulation and coding scheme (MCS) field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating redundancy versions (RVs) 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
DRX (Discontinuous Reception)
DRX functionality can be configured for RRC_IDLE, in which case the UE uses either the specific or default DRX value (defaultPagingCycle); the default is broadcasted in the System Information, and can have values of 32, 64, 128 and 256 radio frames. If both specific and default values are available, the shorter value of the two is chosen by the UE. The UE needs to wake up for one paging occasion per DRX cycle, the paging occasion being one subframe.
DRX functionality can be also configured for an “RRC_CONNECTED” UE, so that it does not always need to monitor the downlink channels. In order to provide reasonable battery consumption of user equipment, 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX). Technical Standard TS 36.321 Chapter 5.7 explains the DRX and is incorporated by reference herein.
The following parameters are available to define the DRX UE behavior; i.e., the On-Duration periods at which the mobile node is active, and the periods where the mobile node is in a DRX mode.
On Duration:                duration in downlink sub-frames that the user equipment, after waking up from DRX, receives and monitors the PDCCH. If the user equipment successfully decodes a PDCCH, the user equipment stays awake and starts the inactivity timer; [1-200 subframes; 16 steps: 1-6, 10-60, 80, 100, 200]        
DRX Inactivity Timer:                duration in downlink sub-frames that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH; when the UE fails to decode a PDCCH during this period, it re-enters DRX. The user equipment shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e., not for retransmissions). [1-2560 subframes; 22 steps, 10 spares: 1-6, 8, 10-60, 80, 100-300, 500, 750, 1280, 1920, 2560]        
DRX Retransmission Timer:                specifies the number of consecutive PDCCH subframes where a downlink retransmission is expected by the UE after the first available retransmission time. [1-33 subframes, 8 steps: 1, 2, 4, 6, 8, 16, 24, 33]        
DRX Short Cycle:                specifies the periodic repetition of the on duration followed by a possible period of inactivity for the short DRX cycle. This parameter is optional. [2-640 subframes; 16 steps: 2, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640]        
DRX Short Cycle Timer:                specifies the number of consecutive subframes the UE follows the short DRX cycle after the DRX Inactivity Timer has expired. This parameter is optional. [1-16 subframes]        
Long DRX Cycle Start Offset:                specifies the periodic repetition of the on duration followed by a possible period of inactivity for the DRX long cycle as well as an offset in subframes when on-duration starts (determined by formula defined in TS 36.321 section 5.7); [cycle length 10-2560 subframes; 16 steps: 10, 20, 30, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640, 1024, 1280, 2048, 2560; offset is an integer between [0-subframe length of chosen cycle]]        
The total duration that the UE is awake is called “Active time”. The Active Time includes the on-duration of the DRX cycle, the time UE is performing continuous reception while the inactivity timer has not expired and the time UE is performing continuous reception while waiting for a downlink retransmission after one HRQ RTT. Similarly, for the uplink the UE is awake at the subframes where uplink retransmission grants can be received, i.e., every 8 ms after initial uplink transmission until maximum number of retransmissions is reached. Based on the above, the minimum active time is of fixed length equal to on-duration, and the maximum is variable depending on, e.g., the PDCCH activity.
The operation of DRX gives the mobile terminal the opportunity to deactivate the radio circuits repeatedly (according to the currently active DRX cycle) in order to save power. Whether the UE indeed remains in DRX (i.e., is not active) during the DRX period may be decided by the UE; for example, the UE usually performs inter-frequency measurements which cannot be conducted during the On-Duration, and thus need to be performed some other time, during the DRX opportunity of time.
The parameterization of the DRX cycle involves a trade-off between battery saving and latency. For example, in case of a web browsing service, it is usually a waste of resources for a UE to continuously receive downlink channels while the user is reading a downloaded web page. On the one hand, a long DRX period is beneficial for lengthening the UE's battery life. On the other hand, a short DRX period is better for faster response when data transfer is resumed—for example when a user requests another web page.
To meet these conflicting requirements, two DRX cycles—a short cycle and a long cycle—can be configured for each UE; the short DRX cycle is optional, i.e., only the long DRX cycle is used. The transition between the short DRX cycle, the long DRX cycle and continuous reception is controlled either by a timer or by explicit commands from the eNodeB. In some sense, the short DRX cycle can be considered as a confirmation period in case a late packet arrives, before the UE enters the long DRX cycle. If data arrives at the eNodeB while the UE is in the short DRX cycle, the data is scheduled for transmission at the next on-duration time, and the UE then resumes continuous reception. On the other hand, if no data arrives at the eNodeB during the short DRX cycle, the UE enters the long DRX cycle, assuming that the packet activity is finished for the time being.
During the Active Time the UE monitors PDCCH, reports SRS (Sounding Reference Signal) as configured and reports CQI (Channel Quality Information)/PMI (Precoding Matrix Indicator)/RI (Rank Indicator)/PTI (Precoder Type Indication) on PUCCH. When UE is not in Active time, type-O-triggered SRS and CQI/PMI/RI/PTI on PUCCH may not be reported. If CQI masking is set up for the UE, the reporting of CQI/PMI/RI/PTI on PUCCH is limited to On Duration. Available DRX values are controlled by the network and start from non-DRX up to x seconds. Value x may be as long as the paging DRX used in RRC_IDLE. Measurement requirements and reporting criteria can differ according to the length of the DRX interval, i.e., long DRX intervals may have more relaxed requirements (for more details see further below). When DRX is configured, periodic CQI reports can only be sent by the UE during “active-time”. RRC can further restrict periodic CQI reports so that they are only sent during the on-duration.
FIG. 8 discloses an example of DRX. The UE checks for scheduling messages (indicated by its C-RNTI, cell radio network temporary identity, on the PDCCH) during the “on duration” period, which is the same for the long DRX cycle and the short DRX cycle. When a scheduling message is received during an “on duration”, the UE starts an “inactivity timer” and monitors the PDCCH in every subframe while the Inactivity Timer is running. During this period, the UE can be regarded as being in a continuous reception mode. Whenever a scheduling message is received while the Inactivity Timer is running, the UE restarts the Inactivity Timer, and when it expires the UE moves into a short DRX cycle and starts a “short DRX cycle timer”. The short DRX cycle may also be initiated by means of a MAC Control Element. When the short DRX cycle timer expires, the UE moves into a long DRX cycle.
In addition to this DRX behavior, a ‘HARQ Round Trip Time (RTT) timer’ is defined with the aim of allowing the UE to sleep during the HARQ RTT. When decoding of a downlink transport block for one HARQ process fails, the UE can assume that the next retransmission of the transport block will occur after at least ‘HARQ RTT’ subframes. While the HARQ RTT timer is running, the UE does not need to monitor the PDCCH. At the expiry of the HARQ RTT timer, the UE resumes reception of the PDCCH as normal.
There is only one DRX cycle per user equipment. All aggregated component carriers follow this DRX pattern.
Machine to Machine
The current mobile networks are optimally designed for Human-to-Human communications, but are less optimal for M2M (Machine-2-Machine) applications, which according to 3GPP is also termed MTC (Machine-Type-Communication).
M2M Communication can be seen as a form of data communication between entities that do not necessarily need human interaction. It is different to current communication models as it involves new or different market scenarios, lower costs and effort, a potentially very large number of communicating terminals and little traffic per terminal to a large extent.
Some MTC applications are for example:                Security (e.g., Alarm Systems, Backup for landline, Access Control, Car/Driver security)        Tracking & Tracing (e.g., Fleet Management, Order Management, Pay as you drive, Road Tolling, Traffic information)        Payment (Point of Sales, Vending machines, Loyalty Concepts, Gaming machines)        Health (Monitoring vital signs, Remote diagnostics, Web Access Telemedicine point)        Remote Maintenance/Control (Sensors, Lighting, Pumps, Valves, Elevator control)        Metering (e.g., Power, Gas, Water, Heating, Grid Control)        
A study item on M2M communications (3GPP TR 22.868) was completed in 2007. For Rel-10 and beyond, 3GPP intends to take the results on network improvements from the study item forward into a specification phase and address the architectural impacts and security aspects to support MTC scenarios and applications. As such, 3GPP has defined a work item on Network Improvements for Machine-Type Communication (NIMTC) with different goals and objectives such as to reduce the impact and effort of handling large machine-type communication groups, optimize network operations to minimize impact on device battery power usage, stimulate new machine-type communication applications by enabling operators to offer services tailored to machine-type communication requirements or provide network operators with lower operational costs when offering machine-type communication services.
The MTC has some specifics that are different from the usual human-to-human communication. 3GPP tries to identify these specifics in order to optimize the network operations. These specifics are called “MTC features” and are explained in the technical standard TS 22.368 available from http://www.3gpp.org and incorporated herein by reference. For example, one of the mentioned MTC feature can be “small data transmissions”, meaning that the MTC device sends or receives small amounts of data. Small amount of data means that the data is smaller or comparable with the size of the exchanged signaling needed to establish the data connection.
Extra low-power consumption is also considered as a critical requirement for some types of MTC devices. For some devices, such as those used for gas metering and animal, cargo, prisoner, elderly and children tracking, low power consumption is critical because it is not easy to recharge or replace the battery. This creates the need for enhancements that would minimize the power consumption of MTC devices. Enhancements for optimizing battery consumption can be foreseen on the architecture level as well as on a lower layer protocol level such as PHY/MAC.
Moreover, LTE RAN enhancements for diverse data applications are under study in 3GPP. The machine type communication traffic profiles include sporadic data access for exchange of relatively small data amounts. Such a type of communication is particularly relevant for applications which require always-on connectivity, such as smart phones, sporadic access for the purpose of checking e-mails or social network updates. The aim of the working items is to identify and specify mechanisms at the radio access network level that enable enhancing the ability of the LTE to handle diverse traffic profiles. In particular, the aim is to reduce the power usage of the terminals in order to extend the battery life. The machine type communication traffic is in general delay insensitive, in which terminals and/or eNodeB can wait for some time until the data is delivered.
Uplink Access Scheme for LTE
For Uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, Node B assigns users a unique time/frequency resource for transmitting user data thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g., a sub-frame of 0.5 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
Uplink Scheduling Scheme for LTE
The uplink scheme allows for both scheduled access, i.e., controlled by eNB, and contention-based access.
In case of scheduled access, the UE is allocated a certain frequency resource for a certain time (i.e., a time/frequency resource) for uplink data transmission. However, some time/frequency resources can be allocated for contention-based access; within these time/frequency resources, UEs can transmit without first being scheduled. One scenario where UE is making a contention-based access is for example the random access, i.e., when UE is performing initial access to a cell or for requesting uplink resources.
For the scheduled access the Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically the scheduler determines                which UE(s) that is (are) allowed to transmit,        which physical channel resources (frequency),        Transport format (Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission        
The allocation information is signaled to the UE via a scheduling grant, sent on the L1/L2 control channel. For simplicity reasons this channel may be called uplink grant channel in the following. A scheduling grant message contains at least information which part of the frequency band the UE is allowed to use, the validity period of the grant and the transport format the UE has to use for the upcoming uplink transmission. The shortest validity period is one subframe. Additional information may also be included in the grant message, depending on the selected scheme. Only “per UE” grants are used to grant the right to transmit on the UL-SCH (i.e., there are no “per UE per RB” grants). Therefore, the UE needs to distribute the allocated resources among the radio bearers according to some rules. Unlike in HSUPA there is no UE-based transport format selection. The eNB decides the transport format based on some information, e.g., reported scheduling information and QoS info, and UE has to follow the selected transport format. In HSUPA the Node B assigns the maximum uplink resource, and the UE selects accordingly the actual transport format for the data transmissions.
Since the scheduling of radio resources is the most important function in a shared channel access network for determining Quality of service, there are a number of requirements that should be fulfilled by the UL scheduling scheme for LTE in order to allow for an efficient QoS management.                Starvation of low priority services should be avoided        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme        The UL reporting should allow fine granular buffer status reports (e.g., per radio bearer or per radio bearer group) in order to allow the eNB scheduler to identify for which Radio Bearer/service data is to be sent        It should be possible to make clear QoS differentiation between services of different users        It should be possible to provide a minimum bit rate per radio bearer        
As can be seen from the above list, one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregated cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signaled from AGW to eNB as described before. An operator can then allocate a certain amount of its aggregated cell capacity to the aggregated traffic associated with radio bearers of a certain QoS class. The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to.
Buffer Status/Scheduling Request Reporting
The usual mode of scheduling is dynamic scheduling, by means of downlink assignment messages for the allocation of downlink transmission resources and uplink grant messages for the allocation of uplink transmission resources; these are usually valid for specific single subframes. They are transmitted on the PDCCH using C-RNTI of the UE as already mentioned before. Dynamic scheduling is efficient for services types, in which the traffic is bursty and dynamic in rate, such as TCP.
In addition to the dynamic scheduling, a persistent scheduling is defined, which enables radio resources to be semi-statically configured and allocated to a UE for a longer time period than one subframe, thus avoiding the need for specific downlink assignment messages or uplink grant messages over the PDCCH for each subframe. Persistent scheduling is useful for services such as VoIP for which the data packets are small, periodic and semi-static in size. Thus, the overhead of the PDCCH is significantly reduced compared to the case of dynamic scheduling.
Buffer status reports (BSR) from the UE to the eNodeB are used to assist the eNodeB in allocating uplink resources, i.e., uplink scheduling. For the downlink case, the eNB scheduler is obviously aware of the amount of data to be delivered to each UE; however, for the uplink direction, since scheduling decisions are done at the eNB and the buffer for the data is in the UE, BSRs have to be sent from the UE to the eNB in order to indicate the amount of data that needs to be transmitted over the UL-SCH.
Buffer Status Report MAC control elements for LTE consist of either: a long BSR (with four buffer size fields corresponding to LCG IDs #0-3) or a short BSR (with one LCG ID field and one corresponding buffer size field). The buffer size field indicates the total amount of data available across all logical channels of a logical channel group, and is indicated in number of bytes encoded as an index of different buffer size levels (see also 3GPP TS 36.321 v 10.5.0 Chapter 6.1.3.1, incorporated herewith by reference).
Which one of either the short or the long BSR is transmitted by the UE depends on the available transmission resources in a transport block, on how many groups of logical channels have non-empty buffers and on whether a specific event is triggered at the UE. The long BSR reports the amount of data for four logical channel groups, whereas the short BSR indicates the amount of data buffered for only the highest logical channel group.
The reason for introducing the logical channel group concept is that even though the UE may have more than four logical channels configured, reporting the buffer status for each individual logical channel would cause too much signaling overhead. Therefore, the eNB assigns each logical channel to a logical channel group; preferably, logical channels with same/similar QoS requirements should be allocated within the same logical channel group.
In order to be robust against transmission failures, there is a BSR retransmission mechanism defined for LTE; the retransmission BSR timer is started or restarted whenever an uplink grant is restarted; if no uplink grant is received before the retransmission BSR timer expires, another BSR is triggered by the UE.
A BSR is triggered for events, such as:                Whenever data arrives for a logical channel, which has a higher priority than the logical channels whose buffer are non-empty (i.e., whose buffer previously contained data);        Whenever data becomes available for any logical channel, when there was previously no data available for transmission (i.e., all buffers previously empty)        Whenever the retransmission BSR time expires        Whenever periodic BSR reporting is due, i.e., periodicBSR timer expires        Whenever there is a spare space in a transport block which can accommodate a BSR        
More detailed information with regard to BSR and in particular the triggering of same is explained in 3GPP TS 36.321 V10.5 in Chapter 5.4.5 incorporated herewith by reference.
If the UE has no uplink resources allocated for including a BSR in the transport block when a BSR is triggered, the UE sends a scheduling request (SR) to the eNodeB so as to be allocated with uplink resources to transmit the BSR. Either a single-bit scheduling request is sent over the PUCCH (dedicated scheduling request, D-SR), or the random access procedure is performed to request an allocation of an uplink radio resource for sending a BSR.
However, for sake of completion it should be noted that the UE will not trigger the transmission of the Scheduling Request for the case that a periodic BSR is to be transmitted.
Furthermore, an enhancement to the SR transmission has been introduced for the specific scheduling mode where resources are persistently allocated with a defined periodicity in order to save L1/L2 control signaling overhead for transmission grants, which is referred to as semi-persistent scheduling (SPS). One example for a service, which has been mainly considered for semi-persistent scheduling, is VoIP. Every 20 ms a VoIP packet is generated at the Codec during a talking spurt. Therefore, the eNodeB can allocate uplink or respectively downlink resources persistently every 20 ms, which could then be used for the transmission of VoIP packets. In general, SPS is beneficial for services with predictable traffic behavior, i.e., constant bit rate, packet arrival time is periodic. For the case where SPS is configured for the uplink direction, the eNodeB can turn off SR triggering/transmission for certain configured logical channels, i.e., BSR triggering due to data arrival on those specific configured logical channels will not trigger an SR. The motivation for such kind of enhancements is that sending a Scheduling Request for those logical channels which will use the semi-persistently allocated resources (logical channels which carry VoIP packets) is of no value for eNB scheduling and hence should be avoided.
Disadvantages of the Prior Art
A current work item in LTE relates to RAN Enhancements for Diverse Data Applications. The idea is to enhance current mobile networks to better support smartphones, laptops, netbooks, tablets and embedded modems running a wide variety of data applications, often in parallel. The main goal is to identify and specify mechanisms at the radio access network level that enhance the ability of LTE to handle diverse traffic profiles. The diversity and unpredictable nature of application traffic profiles leads to challenges in optimizing the network and in guaranteeing efficient operation under all use cases. RRC state control mechanisms and DRX configurations may be optimized with particular applications in mind, but these may not remain optimal as different applications are installed/started/stopped on the device and as the consequent traffic profile of the device changes over time. A further aim is to reduce the power usage of the terminals in order to extend the battery life. Also, for certain MTC devices low power consumption might be a very critical requirement.
The currently standardized reporting of the buffer status report and the scheduling request is not power efficient, as will be explained in the following with reference to FIG. 9.
FIG. 9 illustrates in an exemplary way the UE behavior relating to BSR/SR when data arrives in the transmission buffer of the UE (UE Tx buffer). For explanatory purposes the following somewhat simplified scenario is assumed. Only one transmission buffer and one logical channel of a UE is considered. The transmission buffer is assumed to be empty at the beginning, i.e., no data is stored in the transmission buffer. Furthermore, the UE shall not have sufficient uplink resources to transmit a buffer status report to the eNodeB. However, the UE shall have semi-statically (by means of RRC signaling) allocated resources available in the PUCCH for transmitting a scheduling request (also referred to as dedicated scheduling request, D-SR), when necessary.
Of course, the problem applies correspondingly to transmission buffers of other logical channels, as well as to a logical channel group where the transmission buffers of the logical channels grouped into the logical channel group are considered together. Also, the transmission buffer(s) need not be empty; in said case however, the new data (i.e., the data that currently arrives) entering the transmission buffer of the UE shall have a higher priority than the data already previously stored in the transmission buffer. Instead of using the allocated resources of the PUCCH for transmitting the SR, the UE might have to perform a RACH procedure to transmit the scheduling request in case no such D-SR uplink resources are available.
When new data arrives in the transmission buffer of the UE at time t1, the UE has to first request uplink resources for transmission of the data since no appropriate uplink resources are momentarily available in said respect. Thus, according to the standard trigger condition as explained above, a BSR is triggered in the UE, and in view of the lack of uplink resources for transmitting even the BSR, a scheduling request is triggered in the UE for transmission.
The UE uses the allocated PUCCH resources (or RACH procedure not shown in FIG. 9) to transmit the scheduling request to the eNodeB so as to request the eNodeB to allocate uplink resources to the UE. Accordingly, the eNodeB allocates some UL-SCH resources to the UE. Depending, e.g., on the current resource usage in the uplink, the eNodeB may allocate less or more uplink resources to the UE in response to the SR, and will transmit a corresponding uplink grant via the PDCCH.
Upon receiving the uplink grant message, the UE may or may not transmit data in addition to the BSR, depending on the amount of allocated PUSCH resources. When generating the BSR, this is considered by the UE, such that the BSR indicates the amount of data in the transmission buffer after transmitting the BSR and possibly data of the transmission buffer.
Thus, the UE will transmit over the PUSCH only the BSR or may also include some data of the UE transmission buffer. In FIG. 9 in the first signaling exchange, it is assumed that the UE can transmit all data of the transmission buffer to the eNodeB using the uplink resources assigned by the eNodeB in response to the SR. Correspondingly, the BSR informs the eNodeB about basically an empty transmission buffer, such that no further uplink grant is necessary to be allocated.
However, usually more than one uplink transmission will be necessary in order to empty the transmission buffer, as is also illustrated in FIG. 9 in connection with new data arriving at time t2. In this case, the amount of data is larger than the data becoming available in the transmission buffer at time t1. However, the above procedure basically repeats itself as illustrated, with the exception that the PUSCH transmission, including the BSR and the data, does not suffice to empty the transmission buffer. Correspondingly, the BSR generated by the UE at the time of transmission informs the eNodeB about the remaining data in the transmission buffer. The eNodeB thus will allocate to the UE further uplink resources in correspondence with the remaining data in the transmission buffer. An uplink grant message is transmitted by the eNodeB to the UE, which in turn can then use the assigned uplink resources to transmit the remaining data and thus empty its transmission buffer.
Each time new data arrives in the buffer, one of the above procedure will be repeated. Hence, e.g., for low-volume background services/Instant Messaging services where only a small amount of data needs to be transmitted in certain frequent intervals (though intervals does not need to be periodically) the currently defined BSR/SR procedure as of FIG. 9 is not efficient from the transmission power point of view. The frequent transmissions of SR/BSR consumes a lot of transmit power at the UE. Further, also the PDCCH overhead (i.e., uplink grants) cannot be neglected since the eNodeB needs to issue many PDCCHs in order to schedule frequent PUSCH transmissions.